For the tree to read nested data, the Ext.data.reader.Reader must be configured with a root property, so the reader can find nested data for each node (if a root is not specified, it will default to 'children'). This will tell the tree to look for any nested tree nodes by the same keyword, i.e., 'children'. If a root is specified in the config make sure that any nested nodes with children have the same name. Note that setting defaultRootProperty accomplishes the same thing.